Hollow, plastic containers are typically manufactured using a blow mold machine. In general, there are two types of machines: a two-stage, reheat, stretch blow mold machine and a one-stage blow mold machine. Typically a stretch blow mold machine consumes a plastic preform created utilizing an extruder, in an injection-molding operation. Alternatively, a one-stage blow mold machine uses a parison formed by an extruder in a more continuous process. That is to say the preform is injection molded and can be warehoused, whereas the parison is formed just before it is blown.
Functionally, the one-stage blow mold machine differs from the following discussion of two-stage machines in that the parison is typically formed just before it is transformed into the container and there is no stretch rod. On two-stage or reheat-stretch machines, the preforms are conveyed to the stretch blow mold machine and often times inspected by automated visual scanners to detect irregularities. Any preforms failing to meet preprogrammed criterion are rejected before being loaded into the stretch blow mold machine. In general, the first step of the stretch blow mold process is to reheat the preform in an oven upstream from the stretch blow mold machine rotary table. The preforms are heated as they are conveyed through the oven. Typically, the heated preform is transferred to the rotary stretch blow mold machine at the load station. The load station includes rotating arms that grasp the preform and place the preform in the two-piece mold cavity. The load station is synchronized with the stretch blow mold machine wheel and the mold closes around the preform after the preform is loaded.
As the wheel continues to rotate, the stretch rod is driven down through the neck of the preform. When the stretch rod is moving down, low-pressure air (typically 150 psi) is introduced into the preform via the hollow stretch rod to prevent the preform from collapsing on the stretch rod. Some time after the stretch rod reaches its maximum extension or stroke, high pressure blow air (typically 600 psi) is applied to the inside of the preform to force the heated plastic against the mold surrounding the preform. It is understood that the container may be formed by any fluid, not only air. The mold is typically cooled by chilled process water circulated in the jacketed mold. The high-pressure blow air is applied to the now formed container for a portion of the wheel's revolution to allow the container to cool.
After the cooling interval, the center rod retracts and the blow air is exhausted. The stretch rod retracts to the fully retracted position before the mold opens to expose the container for extraction. The extraction or unload station is synchronized to the stretch blow mold machine wheel and transfers the container from the wheel to the exit conveyor. The exit conveyor transports the container to an approval inspection station and then on to a palletizer.
Stretch blow mold machines are available in many configurations ranging from two stations to 40 or more stations. Previously, there has been no way to monitor specific process parameters at the individual stations used to produce a container. Previous systems were only able to monitor process parameters as they related to all stations on a wheel. Blow air pressure and water values could only be determined at the manifold level. There was no method to monitor the displacement of the stretch rod. Station molds, stretch rods, valves, seals, cams, ports, orifices, and other unique components wear at different rates, are subject to different alignment errors during normal operation, mold changes and routine maintenance activities. These differences affect the quality of containers produced. Significant station-to-station differences are difficult or impossible to detect during continuous production. Quality problems are detected only during random quality control samples. If a bad container is found during this quality check, it is impossible to determine how many out of specification containers have been produced.
A stretch blow mold machine may produce as many as 40,000 or more containers per hour. The wheel is totally enclosed by safety walls and doors. The high angular velocities make it difficult to visually observe station actions without the use of a strobe light. Very small process changes negatively affect the quality of the container. Most of these process changes are not detectable external to the machine. Even if slip rings were employed to provide power to the wheel, communicating across these rings severely limits the data throughput due to the high noise margin induced by slip ring brushes.
Rotating platforms for container formations create an unusually difficult environment to monitor and control the process. This environment not only challenges existing control methods; but, makes it impossible to communicate large volumes of high-resolution data necessary to visualize and control the process.
A need has thus arisen for a system for monitoring and controlling workstation parameters of a blow mold machine.